looking at LS3 flywheels it looks like ring gear can only go on front of flywheel, so you are limited to block (lower) mounted starter. top mount wont engage ring gear.

Thank you wery much for this advices!!!
First will wait that engine arrive and search flywheel and damper plate later. Only 50 km from me in Austria it is service for PCM and Indmar. Will visit them for this isue, perhaps with engine on trailer.
Is it flex plate to light to not be suitable for marine use?
(Foto on left bottom.) Flexplate For all LS Series Engines | Partsworld Performance

You need the rotating mass of either the flywheel or the torque converter to keep engine spinning smoothly at low rpm. Otherwise each compression will slow engine down and each fire will speed engine up. Plus, put it in gear and it will stall. Flywheels. I don't know if the LS engine is external or internal balance. Fairly sure external. I also don't know if balance changes with engine displacement. I THINK it does. So the different stroke cranks and different weight pistons will-I believe-want a different balance flywheel. EDIT: OK, apparently these are all internal balance engines. on some corvettes they fine tuned the balance while they didn't bother on Camaros. so some will look like external balance. some of these ebay flywheels say ls1,2,3,7 all the same. some say ls2,3,7 need different flywheel from ls1. so, take your time, do some homework.

Nice homework, will do it.

At the end of description by Technical information write: Balanced: Internal
http://www.partsworldperformance.com...ngine-19301358